1. Field of the Invention
The present invention relates to a digital TV receiver, and more particularly, to a carrier recovery device of a digital TV receiver that can stand a multiple path noise.
2. Discussion of the Related Art
Recently, the VSB (Vestigial SideBand) system, employed as a broadcasting standard of a digital TV (hereinafter, DTV) in Korea and U.S.A. presently, is designed to transmit a broadcasting signal by using a frequency assigned to the existing analog TV broadcasting system. However, in order to minimize the influence on the existing analog TV broadcasting system, an intensity of a DTV signal is very low as compared to an intensity of an analog TV signal. Of course, the standard is determined such that there is no problem in reception of the DTV signal even if the intensity of the signal is low by using a variety of coding systems in the DTV signal and channel equalizers for reduction of the influence from a noise. However, if the signal cannot be received properly if a transmission channel condition is very poor.
In general, because the DTV transmission system has a merit in that you can watch a picture having no noise at all as noise occurred on the transmission channel is removed perfectly in reception of the broadcasting signal, but has a demerit in that you can not watch the picture at all if you can not recover the transmission signal fully, it is required that the receiver can receive all signals even if the signals have passed through any level of poor transmission channel.
Hereinafter, the related art DTV receiver will be described with reference to the accompanying drawings. FIG. 1 is a block diagram illustrating the related art DTV receiver.
Referring to FIG. 1, the related art DTV receiver receives a Radio Frequency (RF) signal modulated in a VSB mode through an antenna 101. Then, a tuner 102 selects a particular channel frequency that a user desired, and then the tuner 102 transits a VSB signal of a RF band recorded in the particular channel frequency to an intermediate frequency band (IF; in case of 44 MHz or analog TV broadcasting system, 43.75 MHz is used generally), and filters the signals of the other channels properly.
Also, an output signal of the tuner 102, converting a spectrum of a predetermined channel to a passband signal of IF, passes through a Surface Acoustic Wave (SAW) filter 102 for removing peripheral channel signals and noise signals. For example, since a digital broadcasting signal has information between an intermediate frequency of 44 MHz and a band of 6 MHz, the SAW filter 103 removes all remaining blocks except the band of 6 MHz where information from the output of the tuner 102 exists, and outputs the result to an intermediate frequency amplifier 104.
The intermediate frequency amplifier 104 multiplies the signal outputted from the SAW filter 103 by a previously measured gain value so as to maintain the intensity of the signal outputted to an A/D converter 105 constantly. That is, to maintain the intensity of an input signal of the A/D converter 105 constantly, it is required to control the gain value of the input signal in the intermediate frequency amplifier 104. In this case, the information regarding the input signal may be directly extracted from an analog signal, or from a digital block at the rear of the A/D converter 105.
The signal inputted to the A/D converter 105 is a passband signal of 6 MHz. Accordingly, in the intermediate frequency amplifier 104, the gain value is controlled to maintain the constant intensity to all signals of 6 MHz inputted to the A/D converter 105. Thus, the A/D converter 105 receives and digitizes the signals having the constant intensity from the intermediate frequency amplifier 104. Then, the passband signal digitized in the A/D converter 105 is transit to a base band signal in a carrier recovery unit 106, and then the base band signal is outputted to a DC remover 107. At this time, a carrier signal used for recovering a carrier in the carrier recovery unit 106 is converted to a DC component having a frequency of 0 Hz after recovering the carrier.
That is, for recovering the carrier in the carrier recovery unit 106, the DC component is forcibly inserted into a transmitting signal in a transmitting unit. In this respect, the DC component is not required after recovering the carrier. Accordingly, the DC remover 107 extracts and removes the DC component from the base band signal outputted from the carrier recovery unit 106. Then, the digital signal of the base band having no DC component therein is outputted to a synchronizing unit 108 and a channel equalizer 109.
Generally, noticeable features of the VSB transmission system suggested from Grand Alliances (GA) compared to other DTV transmission systems are a pilot signal, a data segment synchronizing signal, a data segment synchronizing signal, and a field synchronizing signal. Those signals are transmitted from the transmitting unit for improving characteristics of carrier recovery, timing recovery, and the like.
Accordingly, the synchronizing unit 108 restores the data segment synchronizing signal and the field synchronizing signal, which are inserted during transmitting, from the signal having no DC component therein. The restored signals are outputted to the channel equalizer 109, a phase compensator 110, and an FEC (forward error correcting) unit 111.
The channel equalizer 109 removes a linear distortion of amplitude generating interference between symbols, and ghost generated by reflection on building or mountains, and then outputs the result to the phase compensator 110. After that, the phase compensator 110 removes the residual phase noise generated in the tuner 102 from the output signal of the channel equalizer 109, and then outputs the result to the FEC unit 111. Then, the FEC unit 111 restores the transmitting symbol from the receiving signal having no phase noise by using the synchronizing signals, and then outputs the result as a transporting stream type.
At this time, as shown in FIG. 1, the signal completing the analog process is converted to the digital signal in the A/D converter 105, and then is outputted to the carrier recovery unit 106. Thus, in case the carrier is not recovered in the carrier recovery unit 106, the digital blocks provided at the rear of the carrier recovery unit 106 are not operated properly.
FIG. 2 is a spectrum illustrating frequency characteristics of a general air signal. More particularly, FIG. 2 illustrates frequency characteristics of an air signal defined according to the DTV broadcasting standard of Korea and U.S.A. presently. At this time, each channel has different center frequency (fc) and pilot frequency (fp). Herein, the center frequency is referred to as fc, and the pilot frequency is referred to as fp.
Generally, in case of a bandwidth in each terrestrial channel, the center frequency (fc) is the most intermediate frequency of 6M Hz, and the pilot frequency (fp) is the frequency where the carrier signal exists on the transmitting signal. At this time, the term of the pilot frequency is used instead of the carrier since the intensity of the carrier signal is decreased greatly (about 13 dB) and then is transmitted so as to prevent the influence of the DTV signal on the existing analog TV signal. Accordingly, in the carrier recovery unit 106 of the DTV receiver, the location of the pilot frequency (fp) existing on the frequency of the transmitting signal is correctly restored, and then is converted to the base band signal.
As shown in FIG. 3, the most general algorithm of the carrier recovery unit 106 is a Frequency Phase Locked Loop (FLPP), fabrication of which circuit is simple and has an excellent performance to use widely. That is, the carrier recovery unit 106 of the FFLL demodulates I, Q signals of the passband from the A/D converter 105 into I, Q signals of the baseband, and then locks a frequency and a phase.
Hereinafter, the carrier recovery unit 106 of the DTV receiver will be described in more detail. FIG. 3 is a detailed view illustrating the carrier recovery unit of FIG. 1.
Referring to FIG. 3, I, Q signals of the passband digitized in the A/D converter 105 are inputted to a complex multiplier 301. The complex multiplier 301 receives a complex carrier of sine wave and cosine wave, in which the carrier is recovered, through a Numerically Controlled Oscillator (NCO) 308, and then transits I, Q signals of the passband to I, Q signals of the base band by multiplying I, Q signals of the passband outputted through the AID converter 105.
Then, I, Q signals of the base band are outputted to the DC remover 107. Simultaneously, I signal of the base band is outputted to a first low pass filter 302, and Q signal of the base band is outputted to a second low pass filter 303. At this time, the carrier recovery unit 106 requires only signals in the periphery of the frequency where the pilot frequency (fp) exists on a bandwidth of 6 MHz.
That is, in case of I, Q signals of the base band, the pilot signal is converted to the DC component. Particularly, the pilot signal is converted to the frequency component of the periphery of the DC component. This is generated according to the difference between the carrier frequency component of the input signal and the carrier frequency component generated in the NCO 308.
Accordingly, it is possible to recover the carrier by using the component existing in the periphery of the DC component. Also, the remaining data components except the signal component of the periphery of the DC component are removed in the first and second low pass filters 302 and 303 since it is possible to recover the carrier with the signal component of the periphery of the DC component. The output of the first low pass filter 302 is inputted to a delay unit 303. The delay unit 303 delays ILPF(t), I signal from which the data component is removed, for a predetermined time period, and outputs to a symbol extractor 304. In this case, if the pilot component is not correctly converted to the DC component when I signal of the pilot component outputted from the first low pass filter 302 passes through the delay unit 303, it generates a corresponding phase error.
Accordingly, the delay unit 303 converts the difference between the pilot frequency component of the input passband signal and the carrier frequency component of the NCO 308 to the phase error type, and then outputs the result to the symbol extractor 304. The symbol extractor 304 extracts only symbols of the signals outputted from the delay unit 303, and then outputs the extracted symbols to a multiplier 306. Then, the multiplier 306 multiplies the symbol of I signal by qLPF(t), Q signal from which the data component is removed, and then outputs the result of the phase error to a Loop filter 307. The Loop filter 307 filters and adds the phase error, and then outputs the result to the NCO 308. Then, the NCO 308 generates the complex carrier (COS, SIN) in proportion to the output of the Loop filter 307, and outputs the complex carrier to the complex multiplier 301. Accordingly, the complex carrier (COS, SIN) becomes the signal similar to the carrier frequency component of the input signal.
According to repetition of the aforementioned process, the complex carrier signal that is similar to the carrier frequency component of the input signal is generated in the NCO 308, and is outputted to the complex multiplier 301, whereby the complex multiplier 301 transits the signal of passband to the desired signal of base band. That is, if the pilot frequency, the carrier signal component that exists on the input passband, is correctly corresponding to the frequency component of the carrier signal generated in the NCO 308, the function of the carrier recovery unit 106 is completed.
In this case, the two carrier signals have the similar frequency components according to the environmental characteristics of the NCO 308 and the transmission line characteristics. However, the frequencies of the two carrier signals are not correctly corresponding to each other. Accordingly, the carrier recovery unit 106 changes the frequencies of the NCO 308 by compensating the frequency components that are not corresponding to each other, whereby the frequencies of the two carrier signals are corresponding to each other.
If the input signal has no linear noise, the capacity of data and the relative intensity of the pilot signal are constant, whereby it has no influence on the carrier recovery unit 106. However, if there is the linear noise in the input signal, the capacity of data and the relative intensity of the pilot signal are changed according to the delay time of the linear noise and the phase difference.
FIG. 4A illustrates the shape of the passband frequency in case the delay time of the noise is corresponding to 1 symbol block when the phase difference is 0°, and FIG. 4B illustrates the shape of the passband frequency in case the delay time of the noise is corresponding to 1 symbol block when the phase difference is 180°. By comparison to the frequency characteristics of FIG. 2, in case of FIG. 4A, the intensity of the pilot signal is relatively greater than the capacity of data. Meanwhile, in case of FIG. 4B, the intensity of the pilot signal is relatively smaller than the capacity of data.
FIG. 5A illustrates the shape of the passband frequency in case the delay time of the noise is corresponding to 10 symbol block when the phase difference is 0°, and FIG. 5B illustrates the shape of the passband frequency in case the delay time of the noise is corresponding to 10 symbol block when the phase difference is 180°. In case of FIG. 5A, the intensity of the pilot signal is relatively greater than the capacity of data. Meanwhile, in case of FIG. 5B, the intensity of the pilot signal is relatively smaller than the capacity of data.
In case of FIG. 4B and FIG. 5B, the intensity of the pilot signal is small, whereby it is impossible to recover the carrier correctly since the information for carrier recovery is lost.
Accordingly, the related art carrier recovery device of the DTV receiver has the following disadvantages.
In the related art carrier recovery device, the intensity of the pilot signal is small, whereby it is impossible to recover the carrier correctly since the information for carrier recovery is lost. In this case, if the intensity of the input signal of the A/D converter 105 increases, it may generate the problem of deteriorating the function of another digital processor.
Also, in case the linear noise exists on the receiving signal, the frequency signal where the carrier signal component exists may be lost. In this case, it is impossible to use a carrier demodulator operated by extracting the frequency component where the carrier signal exists. Thus, the signal completing the analog process is linearly converted to the digital signal in the A/D converter 105, and then the converted digital signal passes through the carrier demodulator. In this respect, the digital signal processor is not operated properly since the carrier is recovered.